1. Field of the Invention
The present invention relates to a photo-detective semiconductor device which is reverse-biased in operation. The device of the specified type is usually called a photodiode or an avalanche photodiode.
2. Description of the Prior Art
The photodiode or the avalanche photodiode is important as a photodetector in an optical communication system. Semiconductor lasers are used as light sources in the optical communication system. Regarding oscillation wavelengths of 0.8 .mu.m-1.4 .mu.m, the semiconductor lasers of, e.g., a GaAs-GaAlAs system and an InGaAsP-InP system are mainly employed.
As the photodetectors for these semiconductor lasers, ones made of the single crystal of Si are well known. However, they do not have a satisfactory sensitivity to light having wavelengths of and above 1 .mu.m. As such photodetector for the optical communication, a photodetector employing a group III-V compound semiconductor material is requested. With the compound semiconductor material, however, it is the present situation that a satisfactory operation cannot be achieved because of, particularly, underdeveloped surface passivation technology.
Photosensors of mesa and planar structures as shown in FIGS. 1a and 1b are typical examples having hitherto been proposed. In the structure of FIG. 1a, a semiconductor layer 2 of a first conductivity type and a semiconductor layer 3 of a second conductivity type are formed on a semiconductor substrate 1, and electrodes 8 and 9 are further disposed. The mesa structure in FIG. 1a is undesirable in practical use in that, since a high electric field is exposed to the end face of a junction, the characteristics of the device are affected by the properties of a surface protective film. On the other hand, the planar structure in FIG. 1b (for example, Japanese Laid-open Patent Application No. 55-132079) is expected to attain a stable operation as compared with the operation of the mesa structure. In the structure of FIG. 1b, an n.sup.+ -type InP layer 2, an InGaAsP layer 3 and an n-type InP layer 4 are formed on an InP semiconductor substrate 1. Shown at numeral 6 is a diffused layer of, e.g., Cd, the diffusion end face of which forms a p-n junction. Numeral 7 designates an insulating layer, and numerals 8 and 9 designate electrodes.